Signalment:  

Seven-month-old, intact female Pomeranian, canine, Canis familiarisThe patient presented to the referring veterinarian at two months of age with generalized non-pruritic alopecia and tiring during play with littermates. Skin scrape was negative and the alopecia cleared with no treatment by five months of age. At that time a physical exam revealed bowing of the forelegs and thickened distal radial growth plates. Serum biochemistry revealed hypocalcemia and vitamin D and calcium supplementation was initiated approximately one week later. Despite increasing Calcitriol and CaCO3 treatment over the next few months (see table below), the patient became increasingly weaker and developed muscle tremors and seizure activity. The patient was transported to the NCSU College of Veterinary Medicine on November 29, 2007 with rigid extension of forelegs and neck and muscle tremors. Her total calcium on presentation was 5.6 mg/dL. Radiographs revealed generalized osteopenia, as well as flaring of the distal ulnar and radial physis and marginated zones of increased lucency with adjacent bands of bone sclerosis (Fig. 2-1). A vertebral body fracture of T11 with cord compression was also detected. Deep pain sensation was absent in both rear limbs and the tail; euthanasia was elected. All other littermates were normal.

Gross Description:  

A retained upper right canine tooth is present and many other teeth appear immature (mixture of deciduous and permanent teeth). The gingiva is diffusely enlarged and swollen and all teeth are easily moveable. All costochondral junctions are enlarged approximately three times normal size, forming what is commonly referred to as a rachitic rosary (Fig. 2-2). A small round hard nodule (0.5 cm) is present in the middle of the left eighth rib (fracture callus). All ribs bend and break easily on manipulation and are readily cut with a knife. T12 vertebra is displaced dorsally but no abnormal movement is noted between T11 and T12. Two bony nodules are present on either side of the dorsal spinous processes overlying the dorsally displaced T12 vertebral body (fracture calluses). The brain and parathyroid glands are grossly normal. 

Histopathologic Description:

Bone, radius: Within the physis, disarray and disorganization (absence of orderly longitudinal columnar alignment) are present throughout the zone of proliferation while the zone of hypertrophy is markedly reduced to segmentally absent (Fig. 2-3). Several infoldings of cartilage are present that extend below the physis into the metaphysis. Within the primary spongiosa increased numbers of osteoclasts (Fig. 2-4) surround trabeculae while osteoblasts lining trabeculae are rare. Many trabeculae are surrounded by a broad band of unmineralized osteoid (wide osteoid seams). Wispy fibrous connective tissue is present between trabeculae in the primary spongiosa (fibrosis). Bony trabeculae within the secondary spongiosa are sparse and thin and do not form anastomoses with other trabeculae.

Morphologic Diagnosis:  

Radius: Failure of endochondral ossification (vitamin D-resistant rickets, Type II)

Lab Results:  

Calcium and Parathyroid Hormone Levels with Initiation of Treatment

DateTreatment	Calcium Blood Level (RR 8.6-11.8 mg/dL)	Parathyroid Hormone Level (RR 27-155 pg/mL)
			Calcitriol (ng/day)	CaCO3 (mg/day)
8-17-07	N/A	N/A	5.6	782.1
8-25-07	150	100	-	-
9-15-07	200	250	6.0	-
9-20-07	250	250	5.1	-
8-17-07	350	250	4.9	-
10-20-07	400	300	5.7	863.8

Condition:  

Rickets

Contributor Comment:  

Rickets is a defect in mineralization of growing bones due to a lack of vitamin D activity. In most species vitamin D is obtained primarily through the diet and metabolized in the liver. It is converted into its active form in the kidney. Low vitamin D activity leads to hypocalcemia and secondary hyperparathyroidism. This hyperparathyroidism causes mineral loss, especially calcium from bone. Rickets occurs when these changes take place during growth. 

Two main forms of vitamin D-resistant rickets are characterized in humans. Type I is an inborn error in conversion of 25-(OH)D₃ to 1,25(OH)₂D₃ due to a deficiency of the renal 1-hydroxylase enzyme. This condition responds to large doses of vitamin D₂ and D₃. Vitamin D-resistant rickets Type II (VDRR II) in humans is an end organ resistance to 1,25(OH)₂D₃ due to an autosomal recessive congenital defect in the vitamin D receptor (VDR) or a site distal to it. This type of rickets has been reported in a few cats, (4,5,6) but has never before been reported in a canine. In one of the feline cases the cat had signs of hypocalcemia, including muscle tremors, similar to the clinical signs in this canine patient.(5) Two other cats had similar radiologic changes of the radius and ulna, as in the present case, as well as similar costochondral junction changes.(4,6) Similarly these cases had no response to high levels of vitamin D or calcium supplementation; (4,6) however, in two of the cases, the cats became normal after physeal closure.(4,5)

Three main intracellular defects have been identified in human cases of VDRR II:

1. Hormone binding defects including decreased number of sites, decreased binding affinity, or complete absence of binding.
2. Deficient nuclear localization in these cases there is normal binding affinity and capacity, but unmeasurable localization to the nucleus. 
3. A post-receptor defect characterized by normal receptors but deficiency in the induction of the 25-(OH)D-24 hydroxylase enzyme in response to 1,25(OH)₂D.(3)

Defects can be detected using in vitro assessment of VDR binding or the subsequent cellular response to VDR binding by 1,25 dihydroxycholecalciferol (1,25(OH)₂D) in cells, typically fibroblasts cultured from skin biopsies, derived from individuals affected by clinical signs of VDRR II and compared with normal controls.(4) A diagnosis of VDRR II was made in the present case based on clinical signs, radiographic findings, biochemistry, parathyroid levels, and the inability of fibroblasts from the skin of the dog to bind 1,25(OH)₂D. A vitamin D receptor defect was verified through genetic testing on cultured fibroblasts from this dog at Stanford University, confirming the diagnosis of VDRR II in this patient. 

This patient also presented with generalized alopecia, as is the case in approximately 50% of human cases of VDRR II.(3) Keratinocytes contain vitamin D receptors and can respond to the 1,25(OH)₂D₃ produced. The alopecia found in VDR deficient patients suggests a biologic role for the VDR in the epidermis, particularly in the hair follicle.(8) In VDR knock out mice models, the mice are fully haired and grossly normal after birth until approximately three months of age, progressing to generalized alopecia by eight months of age.(8) These findings indicate that the prenatal hair growth and development of the epidermis and first hair growth cycle does not require VDR, but is important in the onset of the second hair growth cycle.(8) Examination of the skin in this canine patient revealed large cystic follicles filled with keratin which corresponds to dermal changes noted in human cases and in rodent models of the disease.(8)

Humans with VDRR II, as well as the patient in this case, are born with normal hair, but become alopecic by six months of age and have rachitic changes that are resistant to all forms of vitamin D therapy.(1) Alopecia generally does not improve in human patients, (1) but normal pelage returned in our canine patient after several months. It is not clear if the alopecia is a genetically linked abnormality or related to the effect of 1,25(OH)₂D₃ on the hair follicle.(1) In humans, alopecia seems to be a marker of a more severe form of the disease as judged by the earlier age at presentation, marked clinical aberrations and poor response to therapy.(3)

Therapy in humans begins with very large doses of vitamin D and oral calcium supplements but has had limited success. Refractory cases need long term nocturnal intravenous calcium infusions and these have successfully healed rickets and promoted mineralization in these patients; (2) however, the therapy is cost prohibitive in veterinary cases.

JPC Diagnosis:  

Bone: Physeal dysplasia characterized by disordered columns of chondrocytes with marrow fibrosis

Conference Comment:  

The contributor did a magnificent job of describing the physiology behind vitamin D-resistant rickets. A lack of dietary vitamin D, inadequate absorption of vitamin D from the gastrointestinal system, or a lack of adequate photobiosynthesis of vitamin D can also cause rickets. 

Hypophosphatemia can lead to rickets, and is known as hypophosphatemic vitamin D-resistant rickets (renal hypophosphatemic rickets).(7) Hypophosphatemia, normocalcemia, and decreased renal tubular reabsorption of phosphate are characteristic of hypophosphatemic vitamin D-resistant rickets. Hypophosphatemia is the sequela of inadequate absorption of phosphorus from the gastrointestinal system or decreased/inadequate reabsorption of phosphorous from the renal system.(7)

Gross lesions of rickets are most striking at sites of rapid growth. The metaphyseal and epiphyseal regions of long bones and the costochondral junctions are commonly affected, producing the classic rachitic rosary in affected animals.(7)

The classic histologic appearance of rickets is the disorganization of columns of chondrocytes and persistence of hypertrophic chondrocytes at sites of endochondral ossification, both at the physes and beneath the articular cartilage.(7) The underlying trabecular bone is often disrupted, and irregular tongues of cartilage are often seen in the metaphyses due to disorganized and disrupted endochondral ossification.

References:

1. AL-Khenaizan S, Vitale P: Vitamin D-dependent rickets type II with alopecia: two case reports and review of the literature. Int J Dermatol 42:682-685, 2003
2. Avioli LV and Krane SM: Metabolic Bone Disease and Clinically Related Disorders, 3rd ed., pp. 221, 767-777, Academic Press, San Diego, CA, 1998
3. Favus, MJ: Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 3rd ed., pp. 311-316, Lippincott-Raven, Philadelphia, PA, 1996
4. Godfrey DR, Anderson RM, Barber PJ, Hewison M: Vitamin D-dependent rickets type II in a cat. J Small Anim Pract 46:440-444, 2005
5. Schreiner CA, Nagode LA: Vitamin D-dependent rickets type 2 in a four-month old cat. JAVMA 222:337-339, 2003
6. Tanner E, Langley-Hobbs SJ: Vitamin D-dependent rickets type 2 with characteristic radiographic changes in a 4-month-old kitten. J Feline Med Surg 7:307-311, 2005
7. Thompson K: Diseases of bones and joints. In: Jubb, Kennedy, and Palmer's Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 1, pp. 75-82. WB Saunders, Edinburgh, Scotland, 2007
8. Xie Z, Komuves L, Yu Q-C, Elalieh H, Ng DC, Leary C, Chang S, Crumrine D, Yoshizawa T, Kato S, Bikle DD: Lack of the vitamin D receptor is associated with reduced epidermal differentiation and hair follicle growth. J Invest Dermatol 118:11-16, 2002

Click the slide to view.

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2-1. Distal radius and ulna, dog.

2-2. Thorax, dog.

2-3. Bone, dog.

2-4. Bone, dog

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